What is the test for? If its a day 21 for hormones, it might be to get a result in a different part of your cycle.
If its related to an illness, maybe something came up out of range, and they hope its resolved/improved in 2 weeks time.
Generally, a repeat is needed for a slightly out of range result. If it was worrying or dangerously out of range, they'd normally repeat it sooner. My grandfather needed a repeat when the blood vial (glass then) was dropped and smashed!
You should be able to get a phone consult with either a GP or nurse at the practice, to advise why it needs repeating.